Semiconductor device

ABSTRACT

A current limiting circuit is connected to the gate (input terminal) of an amplifying transistor. The current limiting circuit includes a protecting transistor, a first protecting resistor connecting the drain to the gate of the protecting transistor, and a second protecting resistor connecting the source to the gate of the protecting transistor. The current limiting circuit limits current, so that electric power larger than the maximum electric power allowable for the amplifying transistor does not pass.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a small and light semiconductor device that can be manufactured at low costs, and is highly resistant to destruction even if excessively large electric power is supplied.

2. Background Art

FIG. 22 is a circuit diagram showing a conventional single-stage amplifier. An amplifying transistor 11 is a field effect transistor (FET) used for high-frequency waves of 0.1 GHz to 110 GHz, for example, the 2.1 GHz band. Input signals from the exterior are inputted in the gate (input terminal) of the amplifying transistor 11. The output signals of the amplifying transistor 11 are outputted to an antenna (not shown) from the drain (output terminal) via a matching circuit 27. Generally in communications systems and radars, a plurality of amplifiers are connected to the antenna to output electric waves.

However, if the antenna is positioned in the vicinity of a metal surface, outputted electromagnetic waves may be reflected and fed back to the amplifying transistor 11. In such a case, if large electric power is fed back, the amplifying transistor 11 may be destructed. To prevent this, an isolator 100 is connected to the output side of the amplifying transistor 11 (for example, refer to Japanese Patent Laid-Open No. 4-31782).

FIG. 23 is a graph showing the results of calculations for the input-output characteristics of a conventional amplifier at 2.1 GHz. In FIG. 23, P_(in) denotes input power, P_(o) denotes output power, I_(d) denotes drain current, and I_(g) denotes gate current. The gate width of the amplifying transistor 11 is 1 mm, and the maximum current I_(max) is 400 mA. It is known from these results that an average current of 50 mA/mm flows in the gate when P_(in) exceeds 25 dBm. Since the amplifying transistor 11 is destructed if such a large current flows, input of P_(in) not less than 25 dBm must be prevented.

FIG. 24 is a circuit diagram showing a conventional terminating resistor. An end of the terminating resistor 53 is grounded in terms of high-frequency waves. In microwave-band equipment, the resistance value of the terminating resistor 53 is 50Ω, which is common as impedance.

FIG. 25 is a circuit diagram showing a conventional T-shaped attenuator. First and second resistors 54 and 55 are connected in series. An end of a third resistor 56 is connected to the connecting point of the first resistor 54 and the second resistor 55, and the other end is grounded in terms of high-frequency waves. By selecting the resistance values of the first to third resistors 54 to 56, a desired attenuation can be obtained.

SUMMARY OF THE INVENTION

In the circuit shown in FIG. 22, since the isolator 100 is a magnetic circuit, there is a problem wherein the size and weight are increased, and the magnet is enlarged to treat large electric power, resulting in high costs. There is also a problem wherein an excessively high high-frequency voltage is supplied to the gate by unexpected electromagnetic conduction or the failure of the equipment, and the amplifying transistor 11 is destructed.

Also in the circuits shown in FIGS. 24 and 25 have a problem wherein if high-frequency signals exceeding the allowable power are supplied, the terminating resistor 53 or first to third resistors 54 to 56 are burned and destructed. In order to elevate the allowable power, the sizes of the terminating resistor 53 or first to third resistors 54 to 56 must be enlarged to improve heat dissipation, and there is a problem of high costs due to the elevation of the sizes and weights of the terminating resistor 53 or first to third resistors 54 to 56.

To solve the above-described problems, it is an object of the present invention to provide a small and light semiconductor device that can be manufactured at low costs, and is highly resistant to destruction even if excessively large electric power is supplied.

According to one aspect of the present invention, a semiconductor device comprising: an amplifying transistor and a current limiting circuit connected to the input terminal of said amplifying transistor, wherein said current limiting circuit limits current so that an electric power larger than the maximum electric power allowable to said amplifying transistor does not pass.

According to the present invention, a small and light semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained at low costs.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a semiconductor device according to the first embodiment of the present invention.

FIG. 2 is a top view showing the semiconductor device according to the first embodiment of the present invention.

FIG. 3 is a graph showing the results of calculating the voltage-current characteristics of a current limiting circuit according to the first embodiment of the present invention when the resistance value of the first and second resistors is 50 kΩ.

FIG. 4 is a graph showing the results of calculating the voltage-resistance characteristics of the same current limiting circuit.

FIG. 5 is a graph showing the results of calculating input-output characteristics when a current limiting circuit of the maximum current of 86 mA is connected to an amplifying transistor.

FIG. 6 is a circuit diagram showing a circuit used for calculation.

FIG. 7 is a graph showing the results of calculating input-output characteristics when a current limiting circuit of the maximum current of 26 mA is connected to an amplifying transistor. The gate width of the protecting transistor 13 is 0.06 mm.

FIG. 8 is a graph showing the results of calculating the voltage-current characteristics of a current limiting circuit according to the first embodiment of the present invention when the resistance value of the first resistors is 50 kΩ and the resistance value of second resistors is 150 kΩ.

FIG. 9 is a graph showing the results of calculating the voltage-resistance characteristics of the same current limiting circuit.

FIG. 10 is a circuit diagram showing a semiconductor device according to the second embodiment of the present invention.

FIG. 11 is a top view showing the semiconductor device according to the second embodiment of the present invention.

FIG. 12 is a sectional view showing a current limiting circuit according to the second embodiment of the present invention.

FIG. 13 is a graph showing the voltage-current characteristics of a current limiting circuit according to the second embodiment of the present invention.

FIG. 14 is a sectional view showing a current limiting circuit according to the second embodiment of the present invention.

FIG. 15 is a sectional view showing a current limiting circuit according to the second embodiment of the present invention.

FIG. 16 is a circuit diagram showing a semiconductor device according to the third embodiment of the present invention.

FIG. 17 is a circuit diagram showing a semiconductor device according to the fourth embodiment of the present invention.

FIG. 18 is a circuit diagram showing a semiconductor device according to the fifth embodiment of the present invention.

FIG. 19 is a circuit diagram showing a semiconductor device according to the sixth embodiment of the present invention.

FIG. 20 is a circuit diagram showing a semiconductor device according to the seventh embodiment of the present invention.

FIG. 21 is a circuit diagram showing a semiconductor device according to the eighth embodiment of the present invention.

FIG. 22 is a circuit diagram showing a conventional single-stage amplifier.

FIG. 23 is a graph showing the results of calculations for the input-output characteristics of a conventional amplifier at 2.1 GHz.

FIG. 24 is a circuit diagram showing a conventional terminating resistor.

FIG. 25 is a circuit diagram showing a conventional T-shaped attenuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a circuit diagram showing a semiconductor device according to the first embodiment of the present invention. An amplifying transistor 11 is a field effect transistor used for high-frequency waves not less than 0.1 GHz and not more than 110 GHz, for example, in the 2.1 GHz band. A current limiting circuit 12 is connected to the gate (input terminal) of the amplifying transistor 11. The source of the amplifying transistor 11 is grounded in terms of high-frequency waves, and output signals are outputted from the drain (output terminal) of the amplifying transistor 11.

The current limiting circuit 12 has a protecting transistor 13, a first protecting resistor 14 that connects the source and the gate of the protecting transistor 13, and a second protecting resistor 15 that connects the drain and the gate of the protecting transistor 13.

FIG. 2 is a top view showing the semiconductor device according to the first embodiment of the present invention. Active regions 16 are formed by implanting an impurity into a semiconductor layer. The interiors of the active regions 16 function as transistors. Source electrodes 17 and a drain electrode 18 are ohmically connected to the active regions 16. Gate electrodes 19 are provided between the source electrodes 17 and the drain electrode 18. These source electrodes 17, drain electrode 18, and gate electrodes 19 constitute the amplifying transistor 11.

A source electrode 21 and a drain electrode 22 are ohmically connected to the active regions 16. A gate electrode 23 is provided between the source electrode 21 and the drain electrode 22. These source electrode 21, drain electrode 22, and gate electrode 23 constitute the protecting transistor 13. The source electrode 21 and the gate electrode 23 of the protecting transistor 13 are connected by a first protecting resistor 14 composed of a metal film, and the drain electrode 22 and the gate electrode 23 are connected by a second protecting resistor 15 composed of a metal film. The drain electrode 22 of the protecting transistor 13 is connected to the gate electrodes 19 of the amplifying transistor 11.

FIG. 3 is a graph showing the results of calculating the voltage-current characteristics of a current limiting circuit according to the first embodiment of the present invention when the resistance value of the first and second resistors is 50 kΩ; and FIG. 4 is a graph showing the results of calculating the voltage-resistance characteristics of the same current limiting circuit. If the voltage across the ends of the current limiting circuit 12 is as low as not exceeding 0.4 V, the current limiting circuit 12 exhibits linear voltage-current characteristics, similar to the voltage-current characteristics of a resistor. Here, the resistance value per unit gate width is about 1.3Ω. Specifically, the current flowing through the current limiting circuit 12 is 300 mA/mm or less, the current limiting circuit 12 functions as a resistor of 1.3Ω. On the other hand, when the current exceeds 300 mA/mm, the resistance value of the current limiting circuit 12 is sharply elevated, and current exceeding 420 mA/mm does not flow.

Even when high-frequency signals with an excessive electric power are thus inputted as input signals, the current limiting circuit 12 limits current so that electric power larger than the maximum power allowable to the amplifying transistor 11 does not pass. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained. The current limiting circuit 12 can also be realized to be small and light at low costs.

FIG. 5 is a graph showing the results of calculating input-output characteristics when a current limiting circuit of the maximum current of 86 mA is connected to an amplifying transistor. FIG. 6 is a circuit diagram showing a circuit used for calculation. The circuit has a configuration wherein matching circuits 26 and 27 are connected to the input terminal and the output terminal, respectively, and electric power is supplied from the exterior to the amplifying transistor 11. The gate width of the protecting transistor 13 is 0.2 mm. Since the maximum current of the amplifying transistor 11 is 400 mA, the current limiting circuit 12 limits the current to ⅕ the maximum current of the amplifying transistor 11.

From the results of calculation, no gate current flows even when the input power up to 50 dBm is supplied. Although the amplifying transistor 11 was conventionally destructed when the input power was elevated to 25 dBm, after providing a current limiting circuit 12 the amplifying transistor 11 is not destructed even when the input power is 50 dBm. Therefore, when a current limiting circuit 12 that limits current to ⅕ or lower the maximum current of the amplifying transistor 11 is used, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained.

When impedance Zo is 50Ω and the input power is 50 dBm, the voltage supplied to the current limiting circuit 12 is about +35 V. Therefore, to allow the semiconductor device according to the present invention to function, both the breakdown voltage between the gate and the drain and the breakdown voltage between the gate and the source of the protecting transistor 13 must be at least 35V. This is a value that can be realized in a GaAs transistor. However, if a GaN transistor is used as the protecting transistor 13, higher breakdown voltage, such as 60V or higher, can be realized.

FIG. 7 is a graph showing the results of calculating input-output characteristics when a current limiting circuit of the maximum current of 26 mA is connected to an amplifying transistor. The gate width of the protecting transistor 13 is 0.06 mm. Since the maximum current of the amplifying transistor 11 is 400 mA, the current limiting circuit 12 limits the current to 1/15 the maximum current of the amplifying transistor 11. Since the current limiting circuit 12 limits current to be lower than the current in the example shown in FIG. 5, it functions as a limiter amplifier that limits output power to be constant against input power within a wide range between 16 and 50 dBm.

FIG. 8 is a graph showing the results of calculating the voltage-current characteristics of a current limiting circuit according to the first embodiment of the present invention when the resistance value of the first resistors is 50 kΩ and the resistance value of second resistors is 150 kΩ; and FIG. 9 is a graph showing the results of calculating the Voltage-resistance characteristics of the same current limiting circuit. Even when the resistance value of the first protecting resistor 14 is different from the resistance value of the second protecting resistor 15, current can be similarly limited. However, the current values become asymmetric depending to whether the voltage across the current limiting circuit 12 is positive or negative. Therefore, there is disadvantage wherein direct-current components are strayed by the asymmetry of wave forms when high-frequency power is supplied. Therefore, to obtain a constant output power especially as a limiter amplifier, it is preferable that the resistance values of the first and second resistors 14 and 15 are equalized, and the current flowing into the current limiting circuit 12 is symmetry to the polarity of the voltage supplied to the current limiting circuit 12.

If the current limiting circuit 12 provided in the close vicinity of the gate of the amplifying transistor 11, unnecessary LC parasitic components do not enter between the current limiting circuit 12 and the gate of the amplifying transistor 11, the current of the current limiting circuit 12 becomes the gate current of the amplifying transistor 11 as it is, and high-frequency current can be more limited.

In the first embodiment, although a field effect transistor is used as the amplifying transistor, other types of transistors, such as a bipolar transistor, may also be used. Also in the first embodiment, although a source-grounded transistor is used as the amplifying transistor 11, the present invention is not limited thereto, but a gate-grounded transistor may also be used. In this case, the current limiting circuit 12 is connected to the source of the amplifying transistor 11. If a similar current limiting circuit is connected to the input terminal of other high-frequency circuit, such as a phase shifter and a switch, the other high-frequency circuit can be protected from excessive electric power.

Second Embodiment

FIG. 10 is a circuit diagram showing a semiconductor device according to the second embodiment of the present invention; and FIG. 11 is a top view showing the semiconductor device according to the second embodiment of the present invention. The current limiting circuit 12 has a transmission line model (TLM) structure equivalent to the epitaxial structure of a double hetero PHEMT (pseudomorphic high electron mobility transistor) without a gate. Other configurations are same as the configurations of the first embodiment.

FIG. 12 is a sectional view showing a current limiting circuit according to the second embodiment of the present invention. A buffer layer 32, an n-AlGaAs layer 33, an i-InGaAs layer 34, an n-AlGaAs layer 35, an i-AlGaAs layer 36, an i-GaAs layer 37, and an n-GaAs layer 38 are sequentially laminated on an n-GaAs substrate 31. To the active regions formed in the semiconductor layers, first and second electrodes 39 and 40 are ohmically connected. Specifically, the current limiting circuit has an epitaxial structure that becomes a PHEMT when a gate electrode is added.

FIG. 13 is a graph showing the voltage-current characteristics of a current limiting circuit according to the second embodiment of the present invention. As shown in the graph, the current limiting circuit 12 according to the second embodiment has current limiting characteristics similar to the current limiting characteristics of the first embodiment. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained. The second embodiment has advantages wherein the structure is simpler and the chip area is smaller than those in the first embodiment.

It is known that although the maximum current changes little when the length L_(TLM) is increased, the linear resistance value when the voltage supplied to the current limiting circuit 12 is low is elevated. When input power is small, since the lower the resistance, the lower the passage loss, and lower linear resistance value is desirable, a shorter L_(TLM) is preferable. On the other hand, since higher breakdown voltage of the current limiting circuit 12 is desirable for elevating the maximum allowable power, a longer L_(TLM) is preferable. The optimal length L_(TLM) is determined by the tradeoff of these factors.

As shown in FIG. 14, by forming an n′ layer 41 (high concentration region) in the active region by the implantation of an impurity between the first electrode 39 and the second electrode 40, the electric field can be reduced, and breakdown voltage can be elevated. Furthermore, as shown in FIG. 15, by forming a recess 42 in the active region between the first electrode 39 and the second electrode 40, the linear resistance value can be lowered while elevating breakdown voltage. Also by using a GaN layer as the semiconductor layer, the linear resistance value can be lowered and breakdown voltage can be elevated without enlarging the length L_(TLM).

Although an epitaxial structure of a double hetero HEMT is used in the second embodiment, other epitaxial structures, such as HFET (hetero-structure field effect transistor) and MESFET (metal semiconductor field effect transistor) can also be used.

Third Embodiment

FIG. 16 is a circuit diagram showing a semiconductor device according to the third embodiment of the present invention. A current limiting circuit 12 is connected to the drain (output terminal) of an amplifying transistor 11 via a matching circuit 27. The current limiting circuit 12 limits current so as not to pass an electric power larger than the maximum power allowed to the amplifying transistor 11. The configuration of the current limiting circuit 12 is the same as the current limiting circuit 12 according to the embodiments 1 and 2.

Thereby, since the current limiting circuit 12 limits current even if excessive electric power returns from the output side, no excessive electric power is supplied to the drain of the amplifying transistor 11. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained. In addition, since an isolator, which has been conventionally required can be omitted, the size and weight of the semiconductor device can be reduced, and costs can be lowered.

In the third embodiment, although the current limiting circuit 12 is connected to the outside of the matching circuit 27, the current limiting circuit 12 can be connected to the middle of the matching circuit 27.

Fourth Embodiment

FIG. 17 is a circuit diagram showing a semiconductor device according to the fourth embodiment of the present invention. An end of a ¼ wavelength line 51 is connected to the drain (output terminal) of an amplifying transistor 11 via a matching circuit 27 and a current limiting circuit 12 a. An end of a current limiting circuit 12 b is connected to the other end of the ¼ wavelength line 51, and the other end of the current limiting circuit 12 b is grounded in terms of high-frequency waves. The configuration of the current limiting circuits 12 a and 12 b is the same as the current limiting circuit 12 according to the embodiments 1 and 2.

In the third embodiment, although no return power from the output side passes when the output end is short-circuited or has intermediate impedance, there is possibility that only voltage is supplied and no current is supplied to destruct the amplifying transistor 11, when the output impedance is open and the output of the amplifying transistor 11 is turned off. Whereas, in the fourth embodiment, even when only voltage is supplied, the current limiting circuit 12 b connected to the ¼ wavelength line 51 limits the current. Therefore, output is short-circuited by the ¼ wavelength line 51, and the supply of excessive voltage to the amplifying transistor 11 is prevented.

Fifth Embodiment

FIG. 18 is a circuit diagram showing a semiconductor device according to the fifth embodiment of the present invention. An end of a current limiting circuit 12 is connected to a terminating resistor 53, and the other end of the current limiting circuit 12 is grounded in terms of high-frequency waves. The configuration of the current limiting circuit 12 is the same as the current limiting circuit 12 according to the embodiments 1 and 2. The sum of the linear resistance value of the current limiting circuit 12 and the resistance value R of the terminating resistor 53 is set to agree with the impedance Z_(o).

When the maximum power allowed to the terminating resistor 53 is P_(max) (W), the current I_(TLM) flowing in the current limiting circuit 12 is given by the following Formula 1:

$\begin{matrix} {I_{TLM} < {\frac{\pi}{4}\sqrt{\frac{2P_{MAX}}{R}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$ where π/4 is a factor wherein the fundamental wave component of a square wave is corrected.

If the current limiting circuit 12 is designed so as to satisfy the above-described conditions, the current limiting circuit 12 can limit current so that no electric power larger than the allowable maximum power P_(max) passes through the terminating resistor 53. For example, when the current of the current limiting circuit 12 is 0.157 A for the terminating resistor 53 having a resistance of 50Ω and an allowable power of 1 W, the maximum power supplied to the terminating resistor 53 becomes 1 W. At this time, 100-W high-frequency power having impedance Zo of 50Ω is supplied, the voltage of the current limiting circuit 12 is about 50 V. Consequently, if the breakdown voltage of the current limiting circuit 12 is not lower than 50 V, the terminating resistor 53 is not burned. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained. In the fifth embodiment, although the impedance of the terminating resistor 53 is 50Ω, the present invention is not limited thereto, but the terminating resistor 53 may have any impedance.

Sixth Embodiment

FIG. 19 is a circuit diagram showing a semiconductor device according to the sixth embodiment of the present invention. First and second resistors 54 and 55 are connected in series. An end of a third resistor 56 is connected to the connecting point of the first resistor 54 and the second resistor 55, and the other end is grounded in terms of high-frequency waves. Current limiting circuits 12 a and 12 b are connected to the first resistor 54 and the second resistor 55, respectively. The current limiting circuits 12 a and 12 b have the same configuration as the current limiting circuits according to the first and second embodiments, and limit current so that no power larger than the allowable maximum power passes through the first to third resistors 54 to 56.

Thereby, since current is limited by the current limiting circuits 12 a and 12 b even if high-frequency signals of excessive power are supplied to the input-output terminal, the first to third resistors 54 to 56 become difficult to burn. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained.

The resistance values of the first to third resistors 54 to 56 are set so that desired attenuation and input-output impedance are obtained by adding the linear resistance of the current limiting circuits 12 a and 12 b. Instead of the current limiting circuits 12 a and 12 b, either one of them can also be used.

Seventh Embodiment

FIG. 20 is a circuit diagram showing a semiconductor device according to the seventh embodiment of the present invention. A plurality of amplifying transistors 11 a to 11 c are connected in parallel. A plurality of current limiting circuits 12 a to 12 c are connected to the gates (input terminals) of the amplifying transistors 11 a to 11 c, respectively. Each of the current limiting circuits 12 a to 12 c has the same configuration of the current limiting circuits according to the first and second embodiments, and limits current so that no power larger than the allowable maximum power passes through the corresponding amplifying transistors 11 a to 11 c.

Since each amplifying transistor is thus provided with a current limiting circuit, high-frequency current flowing in the gate of each amplifying transistor can be limited without intervention of unnecessary parasitic components of LC compared with the case wherein current flowing through a plurality of amplifying transistors are collectively limited. Therefore, even high-frequency waves, such as millimeter-waves, can limit high-frequency current flowing through the gate of each amplifying transistor. Thereby, the semiconductor device is highly resistant to destruction even if excessive power is supplied, and can be used as a limiter amplifier at high frequencies.

In the seventh embodiment, although each amplifying transistor is equipped with a current limiting circuit, each group of two or more amplifying transistors may be equipped with a current limiting circuit.

Eighth Embodiment

FIG. 21 is a circuit diagram showing a semiconductor device according to the eighth embodiment of the present invention. A plurality of current limiting circuits 12 a to 12 c are connected to the gate (input terminal) of an amplifying transistor in series. The plurality of current limiting circuits 12 a to 12 c have the same configuration as the current limiting circuits according to the first and second embodiments, and limit current so that no power larger than the allowable maximum power passes through the amplifying transistor 11.

In the eighth embodiment, since three current limiting circuits are connected in series, the semiconductor device can resist to a voltage three times the breakdown voltage of each current limiting circuit. Therefore, a semiconductor device that is highly resistant to destruction even if an excessive electric power is supplied can be obtained.

In the eighth embodiment, although three current limiting circuits are provided, the present invention is not limited thereto, but the provision of two or more current limiting circuits is required.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2007-137127, filed on May 23, 2007 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety. 

1. A semiconductor device comprising: an amplifying transistor having an input terminal, and a current limiting circuit connected to the input terminal of said amplifying transistor, wherein said current limiting circuit limits current so that an electric power larger than a maximum electric power allowable for said amplifying transistor does not pass, and said current limiting circuit includes a protecting transistor having a source, a gate, and a drain, a first protecting resistor connecting the source to the gate of said protecting transistor, and a second protecting resistor connecting the drain to the gate of said protecting transistor.
 2. The semiconductor device according to claim 1, wherein said current limiting circuit functions as a limiter amplifier for limiting output power.
 3. The semiconductor device according to claim 1, wherein said current limiting circuit limits current to not more than ⅕ of a maximum current of said amplifying transistor.
 4. A semiconductor device comprising: a plurality of amplifying transistors having respective input terminals and connected in parallel, and a plurality of current limiting circuits, each current limiting circuit being connected to the input terminal of a respective amplifying transistor, wherein each of the current limiting circuits limits current so that an electric power larger than a maximum electric power allowable for the corresponding amplifying transistor does not pass, and each current limiting circuit includes a respective protecting transistor having a source, a gate, and a drain, a respective first protecting resistor connecting the source to the gate of said respective protecting transistor, and a respective second protecting resistor connecting the drain to the gate of said respective protecting transistor.
 5. A semiconductor device comprising: an amplifying transistor having an input terminal, and a plurality of current limiting circuits, said current limiting circuits being connected in series to the input terminal of said amplifying transistor, wherein said plurality of current limiting circuits limit current so that an electric power larger than a maximum electric power allowable for said amplifying transistor does not pass, and each current limiting circuit includes a protecting transistor having a source, a gate, and a drain, a first protecting resistor connecting the source to the gate of said protecting transistor, and a second protecting resistor connecting the drain to the gate of said protecting transistor.
 6. A semiconductor device comprising: an amplifying transistor having an input terminal, and a current limiting circuit connected to the input terminal of said amplifying transistor, wherein said current limiting circuit limits current so that an electric power larger than a maximum electric power allowable for said amplifying transistor does not pass, and said current limiting circuit consists of an active region in a semiconductor layer, and first and second electrodes spaced apart from each other, each of said first and second electrodes being ohmically connected to said active region.
 7. The semiconductor device according to claim 6, wherein said active region has a dopant impurity concentration and includes a relatively higher dopant impurity concentration region within said active region, between said first electrode and said second electrode.
 8. The semiconductor device according to claim 6, including a recess in said active region between said first electrode and said second electrode.
 9. The semiconductor device according to claim 6, wherein said semiconductor layer is a GaN layer.
 10. A semiconductor device comprising: an amplifying transistor having an input terminal, and a current limiting circuit connected to the input terminal of said amplifying transistor, wherein said current limiting circuit limits current so that an electric power larger than a maximum electric power allowable for said amplifying transistor does not pass, and current flowing through said current limiting circuit is symmetric with respect to polarity of voltage applied to said current limiting circuit. 